1. Field of the Invention
The present invention relates to a method for fabricating a membrane-electrode assembly and fuel cells adopting a membrane-electrode assembly formed by the method and, more particularly, to a method for fabricating a membrane-electrode assembly employing a nanophase catalyzed membrane, in which the size of carbon particles serving as a support catalyst is controlled, and fuel cells with an improved power density which use a membrane-electrode assembly containing the catalyzed membrane formed by the method.
2. Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) emerging as a future generation clean energy source alternative to fossil energy have a high power density and a high energy conversion efficiency. In addition, PEMFCs operate at room temperature and are easy to seal and miniaturize, so that they can be extensively applied to and used in the field of portable electronic devices.
The PEMFC is a power generator for producing direct current through electrochemical reaction of hydrogen and oxygen. The basic structure of such a cell is shown in FIG. 1. Referring to FIG. 1, the PEMFC has a proton exchange membrane 11 interposed between the anode and the cathode to which reaction gases are supplied. The proton exchange membrane 11 is formed of a solid polymer electrolyte (SPE) with a thickness of 50-200 μm. The anode consists of an anode backing layer 14 and a catalyst layer 12 on the anode backing layer 14. The cathode consists of a cathode backing layer 15 and a catalyst layer 13 on the cathode backing layer 15. The anode and cathode backing layers 14 and 15 are formed of carbon cloth or carbon paper. The surfaces of the anode and cathode backing layers 14 and 15 are treated for easy reaction gases or liquid assess, and for water permeability to the proton exchange membrane 11. In FIG. 1, reference numeral 16 denotes a bipolar plate serving as a current collector and having flow pattern for gas supply.
As reaction gases are supplied to a PEMFC having such a structure described above, hydrogen molecules are decomposed into protons and electrons by oxidation reaction in the anode. The produced protons reach the cathode through the proton exchange membrane 11. Meanwhile, in the cathode oxygen molecules take electrons from the anode and are reduced to oxygen ions by reduction. The produced oxygen ions react with hydrogen ions from the anode and produce water.
On the other hand, direct methanol fuel cells (DMFCs) have the same structure as the PEMFC described previously above but use liquid methanol instead of hydrogen as a fuel source. As methanol is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst, and protons, electrons and carbon dioxide are generated. Although the energy efficiency of the DMFC is lower than the PEMFC, use of a liquid fuel in the DMFC makes its application to and use in portable electronic devices easier.
In the previously described fuel cells, as shown in FIG. 2A, a catalyst layer 21 includes carbon particles 22 loaded with catalytic metal particles 23, and a binder (not shown). The carbon particles 22 function as a reaction base for the incoming reaction gases and increase the chance of reaction of the gases. The catalytic metal particles 23 initiate an oxidation/reduction of fuel sources, such as hydrogen, methanol or oxygen. The binder contributes to strengthening the binding force between the catalyst layer and the proton exchange membrane to prevent delamination of the catalyst layer from the electrodes even if the cell is used for a longer period of time.
Carbon particles are loaded with catalytic metal particles typically by a reduction method. According to this reduction method, a compound having a catalytic metal as its cation is reduced such that only the catalytic metal appears on the surface of a carrier such as carbon particles. As a result, the surface of carbon particles is coated with catalytic metals. A method for forming a catalytic layer using such catalytic metal loaded carbon particles will be described below.
First, platinum-on-carbon (Pt/C) is prepared by coating 0.1-μm carbon particles with 2˜5-nm ultrafine catalytic metal particles, for example, platinum particles, and polytetrafluoroethylene is added as a binder. The resulting composition is deposited in a film form, impregnated with an ionomer solution, and dried, thereby resulting in a complete catalytic layer.
However, this method is disadvantageous in that the entire volume of carbon particles cannot be fully utilized because carbon particles are tens to hundreds times larger than catalytic metal particles. Furthermore, another drawback lies in that a contact area between the proton exchange membrane and the catalytic layers, where catalytic reactions take place, is very small. Because the reactions, such as diffusion of reaction gases, or collection of current, take place in the catalyst-and-carbon particles contact surface, such a catalyst structure in which a relatively large carbon particle is loaded with a large number of catalytic particles is considered as being inefficient.
To solve this problem, a method for forming a catalyst layer directly on a proton conductive membrane by a decal process (U.S. Pat. No. 5,234,777), and a method for uniformly dispersing a catalytic metal over the surface of electrodes (Electrochemica Acta., Vol. 42, No. 10, pp. 1587-1593) have been suggested.
To the former, a catalytic layer composition is coated on a support, and then peeled off to obtain a thin catalyst film. The catalyst film is pressed into the surface of a proton exchange membrane to form a complete catalytic layer. However, this method has the following disadvantages. Although the formation of the catalytic layer is effective in increasing the contact area between the catalytic layer and the proton exchange membrane, it is difficult to fully disperse catalyst particles over the surface of the proton exchange membrane because carbon particles serving as a support for the catalyst have a size of a few micrometers. This is evidenced from FIGS. 2A and 2B. To produce an electrode with a higher power density than a predetermined level, the amount of reaction gases for reactions as well as the amount of catalyst must be increased. The increased amount of reaction gases lowers utilization efficiency of the catalyst, and undesirably increases the weight, volume and price of a completed cell. The catalytic layer is bonded to the proton exchange membrane by high-temperature and high-pressure pressing, so that the proton exchange membrane degrades during this hot pressing process.
On the other hand, according to the conventional method for dispersing catalyst metals over the surface of electrodes by sputtering, catalyst utilization efficiency is low because the catalytic metal particles penetrate into porous electrodes. Because the catalytic metal particles are coated on the surface of the carbon particles having a size of a few micrometers, bonding between the catalyst layer and the proton exchange membrane, which allows electrochemical reactions to occur at the contact area, can not be smoothly achieved, so that utilization efficiency of the catalyst becomes low.